Anion complexes anionic adducts

The neutral and anionic adducts of the halides constitute a large proportion of the complexes of Ti, and the alkoxides (also prepared from TiCU) are of commercial importance (p. 968). 

The simplest modification occurs on addition of nucleophilic anions X to Lewis acids MX , resulting in the creation of an ate-complex as anionic adduct (Table 11) (reaction 162). The formation of complex anions can also be the result of halogen addition, with the formation of mixed or symmetrical trihalides [YX2], or else of hydracid addition, with formation of anions [XHY]-, which are not very stable and readily dissociated in solution but are characterizable in the solid state (reaction 163). 

Two of three nitrofluorobenzene isomers react with methoxide, but the third is unreactive. Obtain energies of methoxide anion (at left), ortho, meta and para-nitrofluorobenzene, and the corresponding ortho, meta and para-methoxide anion adducts (so-called Meisenheimer complexes). Calculate the energy of methoxide addition to each of the three substrates. Which substrate is probably unreactive What is the apparent directing effect of a nitro group Does a nitro group have the same effect on nucleophilic aromatic substitution that it has on electrophilic aromatic substitution (see Chapter 13, Problem 4) Examine the structures and electrostatic potential maps of the Meisenheimer complexes. Use resonance arguments to rationalize what you observe. 

The attack of the nucleophile that forms anionic adducts (or zwitterionic adducts, with neutral nucleophiles) parallels the other attacks of nucleophiles on derivatives bearing unsaturated (sp2) carbon atoms, such as alkenes or aromatic substrates, which are both activated by electron-withdrawing groups. The formation of a anionic (or zwitterionic) complexes between nitroaromatic derivatives and nucleophiles is a well-known step in nucleophilic aromatic substitution reactions155,210. 

CPL spectroscopy may also be used to signal the nature of the ternary anion adduct. In the series of phospho-anion complexes of [Eu.la]3+, Figure 5, complexes of HP042 and glucose-6-phosphate give identical CPL spectra - consistent with their very similar NMR spectral profile. Adducts with phospho-tyrosine, N-acetyl phospho-Tyr and a short peptide (phosphorylated at the Pyr residue) are also near-identical, consistent with chemoselective binding of the Tyr-OP phospho-anion. 

Nearly identical spectral changes are observed when GAOX, prepared in a glassing solvent (e.g., 50% glycerol) that prevents formation of microcrystals and preserves the optical transparency of the sample, is cooled to cryogenic temperatures (Fig. 13B) (Whittaker and Whittaker, 1993 Whittaker et aL, 2000). A color change from blue (RT) to red (LT) reflects a thermochromic transition in the protein structure. The similarity of the optical spectrum of the low-temperature complex to the spectra observed for anion adducts suggests that the RT aquo complex (TyroN... 

Pentahalides form adducts with nearly all types of neutral or anionic donors. " The high Lewis acidity of MF5 is illustrated by facile fluoride transfer and formation of anionic adducts. Species such as MF ", n = 1-4) and M2Fn have been detected in HF solutions of the metals using a selective fluoride ion electrode or by F NMR. These complexes... 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

Addition of phenylethynethiolate or -selenate to the carbene complex 185 (M = Cr or W, R = Me or Et, E = S or Se) affords an anionic adduct (186) which, unexpectedly, contains a reactive thioketene function. Electrophiles can attack either at the sulfur or at the carbon a thereto to produce coordinated carbenes or thio- and seleno aldehydes and esters [H4J45). 

The anionic adducts 186 abstract from the disulfide or diselenide, R E E R which adds to the a carbon of the thioketene function, to form the unsaturated dithioester complexes of type 228 84). 

The kinetics of chlorine displacement is strongly dependent on the position of the activating substituent 4-chloronitrobenzene reacts faster than 2-chloronitro-benzene with potassium fluoride whereas, in 2,4-dichloronitrobenzene, chlorine in the 2-position is exchanged more rapidly than chlorine in the 4-position (refs. 4-8). These observations are consistent with an addition-elimination process involving an anionic adduct (Meisenheimer s complex) which has been observed by h and 19F NMR (ref. 9) or UV spectroscopy (ref. 3) ... 

The electronic spectra of the cobalt-substituted derivative are characteristically different when different anions are bound to the metal (Figure 2.37). A catalytically competent ternary complex intermediate displays the electronic absorption pattern typical of anion adducts. ... 

In the TNB-aniline system, a detailed kinetic analysis showed that the lack of formation of a stable a complex in the absence of a tertiary amine catalyst was the result of a thermodynamic, rather than a kinetic, factor (34). That is, the equilibrium constant for formation of the initial zwitterionic adduct (TNB NH2C6H5) is very small, such that in the absence of tertiary amine the overall equilibrium for complex formation is unfavorable. Addition of a tertiary amine, however, causes the equilibrium to be shifted to the right, due to the much more favorable deprotonation of the zwitterionic adduct to the anionic adduct, corresponding to the increased basicity of (CH3CH2)3N or DABCO as compared to C6H5NH2. 

Heating a solution of HjPtCU and an aromatic compound ArH in the CF3COOH-H2O mixture or in CH3COOH leads to the formation of fairly stable CT-aryl complexes of platinum(IV) in yields up to 95%, which can be isolated in the form of anionic adducts with ammonia after chromatography on sihca gel containing ammonia [43, 44] ... 

The complex was proposed to be neutral in order for it to be a radical, but this implies that the initially formed anionic adduct has been oxidised by an external... 

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